Wow, it really depends on the reason for the estrangement, and where people are in their lives at the moment.
My mum and I didn't really communicate in my early 20's. She didn't like some of my choices and I didn't like her criticism. But it wasn't an all out cut off of each other. We slowly came to polite communication, and eventually grew out of the situation.
If you are reaching out with an apology for some past wrong, then it may take some time for it to be accepted. If the reason for the estrangement was built up disagreements, then perhaps a gentle communication with no expected outcome can be sent, and hopefully they will respond in kind.